Disc drives for recording and/or playing back information signals on a disc recording medium are known. Such disc drives have an optical pickup that moves in the radial direction of the disc recording medium, which is loaded on a disc table, to irradiate the disc recording medium with laser light.
The optical pickup has a movable base that moves in the radial direction of the disc recording medium. The movable base includes, for example, predetermined optical components (optical elements and parts). Some optical pickups include a compound lens having several functions for the laser light emitted by a light-emitting device.
Each optical component disposed on the movable base and the optical path of the laser light in a conventional optical pickup including the compound lens will now be described (see FIG. 6).
In the optical pickup, a mounting plate a is disposed on the movable base, which is not shown in the drawing.
This mounting plate a is a thin box having an opening. The mounting plate a has a metal lead frame, not shown in the drawing, for electrically connecting the optical pickup to the outside.
The mounting plate a holds a light-emitting device b. A mounting block c, referred to as a submount, separates the mounting plate a and the light-emitting device b. The light-emitting device b is an edge-emitting device, which emits laser light in the lateral direction.
The mounting plate a holds a reflective mirror d adjacent to the light-emitting device b. The reflective mirror d has a reflective surface e.
The mounting plate a holds a photodetector f on the other side of the reflective mirror d from the light-emitting device b.
The mounting plate a is equipped with a compound lens g covering the light-emitting device b, the reflective mirror d, and the photodetector f. The compound lens g is made of a transparent resin such as polymethyl methacrylate (PMMA).
A transform lens section h and a diffractive section i are separately formed on the top surface of the compound lens g. The transform lens section h is positioned above the reflective surface e of the reflective mirror d; the diffractive section i is positioned above the photodetector f.
A diffractive element j and a focal length adjusting lens section k are separately formed on the bottom surface of the compound lens g. The diffractive element j is positioned below the transform lens section h; the focal length adjusting lens section k is positioned below the diffractive section i.
A beam splitter l is disposed on the other side of the compound lens g from the mounting plate a. The beam splitter l has a splitting surface m.
A reflective prism n is disposed adjacent to the beam splitter l.
A collimator lens o, a waveplate p, and an objective lens q, in that order, are disposed on the other side of the beam splitter l from the compound lens g.
The light-emitting device b emits linearly polarized laser light, which is then reflected by the reflective surface e of the reflective mirror d to be incident on the compound lens g.
The diffractive element j diffracts this laser light incident on the compound lens g into three light beams. The transform lens section h of the compound lens g changes the divergence angle of the diffracted laser light. This process is referred to as numerical aperture (NA) transformation. The resultant laser light is then incident on the beam splitter l.
The laser light incident on the beam splitter l passes through the splitting surface m to be incident on the collimator lens o.
The laser light incident on the collimator lens o is collimated to be incident on the waveplate p, which circularly polarizes the laser light.
The laser light circularly polarized by the waveplate p is incident on the objective lens q, which focuses the laser light on a recording surface of a disc recording medium r.
The recording surface of the disc recording medium r reflects the laser light focused thereon. The reflected laser light then returns through the objective lens q to be incident on the waveplate p, which linearly polarizes the laser light again.
The linearly polarized laser light passes through the collimator lens o to be incident on the beam splitter l. The splitting surface m of the beam splitter l bends the optical path of the laser light 90° to guide the laser light to the reflective prism n.
The optical path of the laser light guided to the reflective prism n is bent 90° by the reflective prism n to be incident on the compound lens g.
The diffractive section i diffracts the laser light incident on the compound lens g into three light beams. The focal length adjusting lens section k adjusts the focal length of the laser light.
The laser light then exits from the compound lens g to be incident on the photodetector f, which performs photoelectric conversion on the laser light. The laser light is output as electrical signals, which, for example, are played back as information signals on the disc recording medium r. The three light beams diffracted by the diffractive element j are used for detecting tracking error signals, while those diffracted by the diffractive section i are used for detecting focusing error signals.
The above compound lens g is a single component that integrates several functions for the laser light. The single component, which has a complicated shape, can readily be made of a resin body. Therefore, the compound lens g can reduce the number of components and production cost of a disc drive.
A compound lens g made of resin unfortunately exhibits changes in its characteristics attributed to changes in its volume and refractive index by variations in temperature and humidity. In particular, aberrations result from such changes in the characteristics during the outward journey of the laser light from the light-emitting device b to the disc recording medium r. These aberrations deteriorate the information signals provided from the disc recording medium r.
For example, a compound lens g made of PMMA in a dry state or in a water-saturated state exhibits changes of up to 0.05% in refractive index and up to 0.04% in thickness. This change in thickness results in a change of up to 0.35% in the optical distance between the light-emitting device b and the objective lens q. For example, a compound lens g with a thickness of 2 mm along the optical axis exhibits a change of up to 7 μm in the optical distance.
Accordingly, an object of the present invention is to provide an optical pickup and a disc drive that solve the above problem, that is, that prevent aberrations of laser light attributed to variations in temperature and humidity, to ensure satisfactory characteristics of the laser light.